The build path is embedded in /usr/lib/apache2/modules/mod_authnz_pam.so:

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/diffoscope-results/libapache2-mod-authnz-pam.html

  /build/1st/libapache2-mod-authnz-pam-1.2.3/mod_authnz_pam.c:332
  vs.
  /build/2/libapache2-mod-authnz-pam-1.2.3/2nd/mod_authnz_pam.c:332

The attached patch to debian/rules fixes this by passing
-ffile-prefix-map to apxs.

According to my local tests, with this patch applied
libapache2-mod-authnz-pam should build reproducibly on
tests.reproducible-builds.org!

Thanks for maintaining libapache2-mod-authnz-pam!

live well,
  vagrant

Dear Maintainer,

Because Debian builds packages from a fixed build path, neither the 'reprotest'
utility in Salsa-CI, nor the Reproducible Builds team's package test
infrastructure for Debian[1] currently check for equivalent binary package
output from differing source package build paths.

This means that your package will pass current reproducibility tests; however
we believe that source code and/or build steps still embed the build path into
the binary package output, making it more difficult than necessary for
independent consumers to check the integrity of those packages by rebuilding
them themselves.

As a result, this bugreport will remain open and be re-assigned the 'wishlist'
severity[2].

For more information about build paths and how they can affect reproducibility,
please refer to: https://reproducible-builds.org/docs/build-path/

Thanks,
James

[1] - https://tests.reproducible-builds.org/debian/reproducible.html

[2] - https://www.debian.org/Bugs/Developer#severities

A correction for a mistake in my previous message:

Currently the 'reprotest' job in Salsa-CI does in fact continue to exercise
variations of the build-path, and will fail if it builds binary packages that
contain different contents as a result.